Nitrogen is an important plant nutrient. In addition to phosphorous, potassium, and other nutrients, nitrogen is needed to support the growth and development of plant life. Some plants, such as legumes, through a symbiotic relationship with Rhizobium bacteria take up elemental nitrogen from the atmosphere and fix this nitrogen into the soil. However, most plants grown to produce human and animal food require the use of nitrogen fertilizer in order to sustain their agricultural production.
The most widely used and agriculturally important high-analysis nitrogen fertilizer is granular urea, CO(NH.sub.2).sub.2. About 60 million tons per year of urea are produced world-wide and used on a variety of crops, such as corn, wheat and rice. When applied to moist soil, the urea becomes a source of ammonia as a result of hydrolysis catalyzed by urease, an enzyme produced by numerous fungi and bacteria. The reaction may be written as follows: ##STR1##
The ammonia formed as shown above undergoes very rapid hydrolysis to form ammonium ions in accordance with the following equilibrium: ##STR2##
In most soils, the ammonium formed through the hydrolysis of urea is readily converted to nitrate via a sequence of bacterial oxidation reactions; the overall oxidation reaction may be written as follows: EQU NH.sub.4.sup.+ +20.sub.2 .fwdarw.NO.sub.3.sup.31 +H.sub.2 O +2H.sup.+
and is commonly referred to as "nitrification".
Both, the ammonium nitrogen derived through the hydrolysis of urea as well as the nitrate nitrogen derived through the oxidation of ammonium may be assimilated directly by the plant. Thus, the urease-catalyzed hydrolysis of urea and the bacterial oxidation of ammonium are two key steps in the vital transformation of urea nitrogen first into ammonium nitrogen and then into nitrate nitrogen, both of which function in soils as nitrogen nutrients.
The major problems associated with the use of urea as a source of these nitrogen nutrients to support the growth of crop plants relate to the fact that the time frame for the catalytic hydrolysis of urea to ammonia and for the subsequent nitrification of ammonium does not coincide with the ongoing demand for nitrogen by the root system of the plants. More specifically, the catalytic hydrolysis of urea and the subsequent nitrification of the ammonium ions proceed relatively rapidly, i.e. within 2 to 20 days, as compared to the 50 to 200 day growing seasons for typical crop plants. Since both ammonia and nitrate can be lost from the soil by various mechanisms before being assimilated by the plant, the premature conversion of urea into ammonium and nitrate nitrogen contributes to the low (40%) efficiency with which crop plants utilize fertilizer nitrogen. Examples of mechanisms by which nitrogen can be lost from the soil include loss of ammonia through volatilization to the atmosphere and loss of nitrate through leaching to the subsoil by rainwater and/or through denitrification, i.e. bacterial conversion of nitrate to elemental nitrogen. Another drawback related to rapid hydrolysis of urea is the potential for excessive accumulation of ammonia in the soil shortly after seeding which may damage germinating seedlings and young plants.
Prior art offers three approaches to make nutrient nitrogen derived from urea-containing granular fertilizers available to root systems of plants throughout their growing season: (1) multiple fertilizer applications, (2) the use of controlled release fertilizers, and (3) the incorporation of urease inhibitors or nitrification inhibitors into the fertilizer formulation. There are certain limitations and disadvantages associated with each of these approaches advocated by prior art.
The first approach involves the use of multiple fertilizer applications during the course of a crop growth season. Such multiple fertilizer applications can provide adequate nitrogen to meet the demand of growing plants, but they do so at the expense of higher fertilizer costs, higher fertilizer application costs, and of the adverse environmental impact associated with the loss of nitrate through leaching to the subsoil.
The second approach to extending the availability of nutrient nitrogen to crop plants over a longer period of time involves the use of controlled release granular fertilizers. The patent literature provides numerous references to prior art methodology for the production of controlled release urea-containing fertilizers. Thus, U.S. Pat. No. 3,295,950 discloses a method of producing sulfur-coated nitrogen fertilizer pellets having a controlled fertilizer dissolution rate. The process comprises applying a coating of sulfur to fertilizer pellets and subsequently top-coating the resultant particles with an oily sealant to impregnate any cracks and voids in said sulfur coating. As a result, a sulfur shell is formed which is nearly impervious to water and suitable for regulating the dissolution rate of the nitrogen fertilizer, and thereby extending its availability to the plants.
U.S. Pat. Nos. 3,576,613; 3,903,333; and 4,081,264 disclose improved processes for sulfur encapsulation of nitrogen fertilizers to provide controlled fertilizer dissolution rates. The '613 patent provides a method for reducing the amount of sulfur needed to attain a coating which is impervious to rain water and which releases nutrients at a rate selected to match the demands of the growing plants. The improvement offered by the '333 patent centers on applying a precisely uniform coating of sulfur onto substrate particles of urea. The process is designed to ensure that the substrate consists of smooth, round granular particles and that each granule is covered by a plurality of sequentially applied streams of fluid (molten) sulfur to form a multitude of thin concentric layers of sulfur coating upon the urea substrate. Finally, the '264 patent discloses a process for preparing a slow release nitrogen fertilizer in which the sulfur coating is impregnated with bitumen and then coated with a mineral powder to produce free flowing particles. In all three cases the desired product is a slow release nitrogen fertilizer, i.e. a material in which the sulfur coating serves to control the release of the substrate to the soil over an extended period of time. U.S. Pat. Nos. 3,877,415 and 3,991,225 disclose equipment suitable for applying a uniform coating of sulfur to urea and thereby imparting slow release characteristics to the coated urea fertilizer. While sulfur-coated urea (SCU) is an article of commerce, its use is very limited. This is a reflection of SCU's premium price and of its lower nitrogen content, as compared to granular urea.
The third approach toward improving the availability of nitrogen to the root system of plants over an extended period of time entails the incorporation of a urease inhibitor or of a nitrification inhibitor into granular urea-containing fertilizers. Urease inhibitors are compounds capable of inhibiting the catalytic activity of the urease enzyme upon urea in moist soil. Among the most effective urease inhibitors are the phosphoric triamide compounds disclosed in U.S. Pat. No. 4,530,714. An example of an effective urease inhibitor disclosed in the '714 patent is N-(n-butyl)thiophosphoric triamide, which will be referred to herein as NBPT. When incorporated into a urea-containing fertilizer, NBPT reduces the rate at which urea is hydrolyzed in the soil to ammonia. The benefits realized as a result of the delayed urea hydrolysis include the following: (1) nutrient nitrogen is available to the plant over a longer period of time, (2) excessive build up of ammonia in the soil following the application of the urea-containing fertilizer is avoided, (3) the potential for nitrogen loss through ammonia volatilization is reduced, (4) the potential for damage by high levels of ammonia to seedlings and young plants is reduced, (5) plant uptake of nitrogen is increased, and (6) an increase in crop yields is attained. While NBPT does not directly influence the rate of ammonium nitrification, it does control the levels of ammonium which are subject to the nitrification process and thereby indirectly controls the levels of nitrate nitrogen in the soil.
NBPT has not been commercially used heretofore as an additive in granular urea, presumably because of the lack of a suitable method for the preparation of such urea-based granular fertilizers stemming from certain physical and chemical characteristics of industrial grade NBPT which render this material difficult to handle. Industrial grade NBPT is a waxy, sticky, heat-sensitive and water-sensitive material. Consequently, the material is susceptible to decomposition during storage and methodology for metering NBPT into continuous production equipment has been heretofore unavailable. Another potential drawback associated with the use of NBPT as a urease inhibitor for granular urea-containing fertilizers is believed to be related to the low ammonium to nitrate ratios which are likely to be observed in soil systems treated with such fertilizers.
The availability of nitrate nitrogen to plants over an extended period of time can also be enhanced through the incorporation of nitrification inhibitors into urea-containing fertilizers. Nitrification inhibitors are compounds capable of inhibiting the bacterial oxidation of ammonium to nitrate in the soil. Among the most effective nitrification inhibitors is dicyandiamide, also referred to as DCD. A granular urea-containing fertilizer formulation containing DCD is disclosed in U.S. Pat. No. 4,994,100. While DCD does not affect the rate at which urea is hydrolyzed to ammonia in the soil, it significantly reduces the rate at which ammonium is oxidized to nitrate. The benefits realized as a result of the delayed nitrification process include the following: (1) nutrient nitrogen is available to the plant over a longer period of time than is the case in the absence of DCD, (2) the potential for loss of nitrate nitrogen through denitrification and/or leaching is reduced, (3) plant uptake of nitrogen is increased, and (4) crop yields are increased. However, the improvement in the performance of DCD-containing granular urea fertilizers attributed to the incorporation of DCD in these formulations is believed to be severely limited by the susceptibility of these formulations to urease-catalyzed hydrolysis following application of the fertilizer to the soil. This may result in relatively high ammonia losses through volatilization and/or in ammonia damage to seedlings and young plants.
In addition to the foregoing, U.S. Pat. Nos. 4,517,003; 4,517,004; 4,932,992; and 4,954,156 make reference to various compounds which are capable of inhibiting both the urease-catalyzed hydrolysis of urea and the oxidation of ammonium to nitrogen. None of these, however, have found commercial acceptance in the fertilizer industry as additives capable of improving the performance of urea-based granular fertilizers in terms of their ability to enhance crop yields.
Accordingly, it is an object of this invention to provide a granular urea-containing fertilizer formulation which offers an effective alternative to the high amounts of urea-containing nitrogen fertilizer that are currently used to assure that crop yields are not limited by the availability of nitrogen as a plant nutrient.
It is a further object of this invention to increase nitrogen uptake efficiency of urea-containing granular fertilizers without the need for applying a substantially impervious sulfur coating to such fertilizers.
It is another object of this invention to provide a urea-based granular fertilizer formulation the performance of which is characterized by relatively low ammonia volatilization losses, low losses of nitrate nitrogen through denitrification and/or leaching, and substantially enhanced crop yields.
It is still another object of this invention to provide a method for the production of the urea-containing granular fertilizer formulations disclosed hereinbelow.